Recent advances in plant genetic engineering have opened new doors to engineer plants to have improved characteristics or traits, such as plant disease resistance, insect resistance, herbicidal resistance, yield improvement, improvement of the nutritional quality of the edible portions of the plant, and enhanced stability or shelf-life of the ultimate consumer product obtained from the plants. Recombinant DNA technology has made it possible to insert DNA sequences at targeted genomic locations and/or modify (edit) specific endogenous chromosomal sequences, thus altering the organism's phenotype. Site-specific integration techniques, which employ site-specific recombination systems, as well as other types of recombination technologies, have been used to generate targeted insertions of genes of interest in a variety of organism. It is important that appropriate regulatory signals must be present in proper configurations in order to obtain the expression of the desired nucleotide sequences in the plant cell. These regulatory signals typically include a promoter region, a 5′ non-translated leader sequence and a 3′ transcription termination/polyadenylation sequence.
A promoter is a non-coding genomic DNA sequence, usually upstream (5′) to the relevant coding sequence, to which RNA polymerase binds before initiating transcription. This binding aligns the RNA polymerase so that transcription will initiate at a specific transcription initiation site. A promoter includes a nucleic acid fragment capable of controlling transcription of another nucleic acid fragment. A promoter is capable of controlling the expression of a coding sequence or functional RNA. Functional RNA includes, but is not limited to, crRNA, tracerRNA, transfer RNA (tRNA) and ribosomal RNA (rRNA). It has been shown that certain promoters are able to direct RNA synthesis at a higher rate than others. These are called “strong promoters”. Certain other promoters have been shown to direct RNA synthesis at higher levels only in particular types of cells or tissues and are often referred to as “tissue specific promoters”, or “tissue-preferred promoters” if the promoters direct RNA synthesis preferably in certain tissues but also in other tissues at reduced levels. Since patterns of expression of a chimeric gene (or genes) introduced into a plant are controlled using promoters, there is an ongoing interest in the isolation of novel promoters which are capable of controlling the expression of a chimeric gene or (genes) at certain levels in specific tissue types or at specific plant developmental stages.
Certain promoters are able to direct RNA synthesis at relatively similar levels across all tissues of a plant. These are called “constitutive promoters” or “tissue-independent” promoters. Constitutive promoters can be divided into strong, moderate and weak according to their effectiveness to direct RNA synthesis. Since it is necessary in many cases to simultaneously express a chimeric gene (or genes) in different tissues of a plant to get the desired functions of the gene (or genes), constitutive promoters are especially useful in this consideration.
Though many constitutive promoters have been discovered from plants and plant viruses and characterized, there is still an ongoing interest in the isolation of more novel constitutive promoters, especially promoters which are capable of controlling the expression functional RNA molecules.